diff --git a/app/components/form/fields/FileField.tsx b/app/components/form/fields/FileField.tsx
index a3e396a14..336704be8 100644
--- a/app/components/form/fields/FileField.tsx
+++ b/app/components/form/fields/FileField.tsx
@@ -9,7 +9,9 @@ import {
   useController,
   type Control,
   type FieldPath,
+  type FieldPathValue,
   type FieldValues,
+  type Validate,
 } from 'react-hook-form'
 
 import { FieldLabel } from '~/ui/lib/FieldLabel'
@@ -30,6 +32,7 @@ export function FileField<
   accept,
   description,
   disabled,
+  validate,
 }: {
   id: string
   name: TName
@@ -40,11 +43,12 @@ export function FileField<
   accept?: string
   description?: string | React.ReactNode
   disabled?: boolean
+  validate?: Validate<FieldPathValue<TFieldValues, TName>, TFieldValues>
 }) {
   const {
     field: { value: _, ...rest },
     fieldState: { error },
-  } = useController({ name, control, rules: { required } })
+  } = useController({ name, control, rules: { required, validate } })
   return (
     <div className="max-w-lg">
       <div className="mb-2">
diff --git a/app/forms/image-upload.tsx b/app/forms/image-upload.tsx
index facc6b28e..a0f4e6086 100644
--- a/app/forms/image-upload.tsx
+++ b/app/forms/image-upload.tsx
@@ -495,7 +495,9 @@ export default function ImageCreate() {
     setAllDone(true)
   }
 
-  const form = useForm({ defaultValues })
+  // onChange mode so the file-size / block-size cross-validation surfaces
+  // inline as soon as the user picks a file or changes block size
+  const form = useForm({ defaultValues, mode: 'onChange' })
   const file = form.watch('imageFile')
   const blockSize = form.watch('blockSize')
 
@@ -605,6 +607,13 @@ export default function ImageCreate() {
           label="Image file"
           required
           control={form.control}
+          // Crucible rejects bulk-write imports whose total size isn't a
+          // multiple of the block size, so catch it before the long upload.
+          validate={(f, { blockSize }) => {
+            if (f && f.size % blockSize !== 0) {
+              return `File size must be a multiple of the block size (${blockSize} bytes)`
+            }
+          }}
         />
         {imageValidation && <BootableNotice {...imageValidation} />}
       </div>
diff --git a/test/e2e/image-upload.e2e.ts b/test/e2e/image-upload.e2e.ts
index cfb3f97e0..17c89a417 100644
--- a/test/e2e/image-upload.e2e.ts
+++ b/test/e2e/image-upload.e2e.ts
@@ -101,6 +101,61 @@ test.describe('Image upload', () => {
     // TODO: changing name alone should cause error to disappear
   })
 
+  test('block size validation', async ({ page, browserName }) => {
+    // eslint-disable-next-line playwright/no-skipped-test
+    test.skip(browserName === 'webkit', 'safari. stop this')
+
+    await page.goto('/projects/mock-project/images-new')
+
+    await page.getByRole('textbox', { name: 'Name' }).fill('new-image')
+    await page.getByRole('textbox', { name: 'Description' }).fill('image description')
+    await page.getByRole('textbox', { name: 'OS' }).fill('Ubuntu')
+    await page.getByRole('textbox', { name: 'Version' }).fill('Dapper Drake')
+
+    const sideModal = page.getByRole('dialog', { name: 'Upload image' })
+    const uploadError = sideModal.getByText(/must be a multiple of the block size/i)
+    const fileRequired = sideModal.getByText('Image file is required')
+    const submit = page.getByRole('button', { name: 'Upload image' })
+    const progressModal = page.getByRole('dialog', { name: 'Image upload progress' })
+
+    // with no file picked, changing block size should not trigger a required
+    // error on the file field
+    await page.getByLabel('4096').click()
+    await expect(fileRequired).toBeHidden()
+    await page.getByLabel('512').click()
+
+    // 1000 bytes is not a multiple of any supported block size (512/2048/4096)
+    await page.getByLabel('Image file').setInputFiles({
+      name: 'my-image.iso',
+      mimeType: 'application/octet-stream',
+      buffer: Buffer.alloc(1000, 'a'),
+    })
+
+    await expect(uploadError).toBeVisible()
+
+    // clicking submit does nothing — validation blocks it and the progress
+    // modal never opens
+    await submit.click()
+    await expect(progressModal).toBeHidden()
+    await expect(uploadError).toBeVisible()
+
+    // replace with an aligned file — error clears
+    await page.getByLabel('Image file').setInputFiles({
+      name: 'my-image.iso',
+      mimeType: 'application/octet-stream',
+      buffer: Buffer.alloc(2048, 'a'),
+    })
+
+    await expect(uploadError).toBeHidden()
+
+    // switching block size to one that no longer divides the file doesn't
+    // surface the error live, but submit-time validation still catches it
+    await page.getByLabel('4096').click()
+    await submit.click()
+    await expect(progressModal).toBeHidden()
+    await expect(uploadError).toBeVisible()
+  })
+
   test('form validation', async ({ page, browserName }) => {
     // eslint-disable-next-line playwright/no-skipped-test
     test.skip(browserName === 'webkit', 'safari. stop this')