How to delete a photo on an iPad after its been transferred to a computer?

To free up space of your iPad, you might want to delete some photos.
The photos you took with your iPad or downloaded from the Internet can be deleted from the Photos app. However, the photos that you synced to iPad with iTunes from your computer's pictures library cannot be removed directly from your iPad.

Step 1. In iTunes, click the Device button in the upper right corner.
Step 2. Click the Photos tab in the resulting window.
Step 3. Choose "Selected albums" and deselect the albums or collections you want to delete.
Note: To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
Step 4. Click Apply.

How to Delete Photos from iPad

Hope this guide helps you. Check it out~
How Can Remove Photos from iPad in Batch

Go into your photo app > Hit select > Select the photo you want to delete (tap on the photo - you should see a check mark on the photo when it is selected) > Select the Trash-Can icon > Select Delete Photo. If you have iOS 8, it may show up in Recently Deleted Photo's list. You can simply delete it from there by repeating the same steps.