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Are these files you use often? Could you store them somewhere like Dropbox. This would:
- Keep them off your iPad, saving storage
- Allow you access whenever you need them (as long as you're on wifi/cellular)
Downside is if you need them frequently you could use up your data.
Another thought -- do you use iCloud Photo Library? This would be more seamless than Dropbox, but you'd still keep most on the iPad. Ones that aren't used frequently would be stored in full on iCloud, with reference photos in your Photos library on the iPad. Those that don't get used frequently, much like with Dropbox, would need to be downloaded, but the positive side is the ones you do tend to use most would most likely stay on the iPad.
Third, if you just want to get those 1,000 or whatever screen shots back, you can sync them with iTunes. Connect your iPad, fire up iTunes, and go to the Photos section.
Inside there it will give you the option to Sync a folder to your iPad (as long as you don't already use iCloud Photo Library). I've used this a good bit in the past with great results. Plus side to this is all photos are on your iPad (no data needed), but that's also the downside as all photos are back on the iPad. iTunes may also optimize the screenshots further, which may affect the file size. I don't think it would upsample them, but may downsample them a bit more than you did already.
Good luck!