Some images I saved simply vanished, any way I can get them back? [4.4.4]

I had a lot of images saved on my Nexus 7 tablet. I had them, I didn't touch them for a few months, then I went back to my Gallery to check them again and a huge chunk of them were gone. I checked every path, they just went up in smokes and I can't understand why, I don't even know when it happened.
They don't simply not appear in my Gallery, the files themselves seem to be nowhere on my device.
I'm not using any external memory slots (well, I can't on the N7), so it's not like an SD got dislodged or something.
I never touched the tablet otherwise, never updated nor tinkered with it in any way for the past two years.
The only problem I encountered is that, a few months ago, some of the pics in the gallery were grayed out, trying to open them would only lead to a gray screen. At first I thought of file corruption, but as far as I know they have the infamous "broken frame" icon instead of being just gray squares, plus it's weird they got deleted by themselves. Rebooting seemed to fix it, the gray, empty pictures had vanished, but I couldn't tell whether or not they disappeared altogether or got restored.
None of my other files went missing, I have half a dozen different albums and they are untouched, only this one (which was the default download folder) had this problem. Plus, I know I didn't move them manually or something like that, so user error is out of the question.
Do you have any idea what might have caused it? I don't think the tablet is failing on me or anything since everything works great, just those specific images of a specific album disappeared and I honestly can't tell how. Is there any way I can try to retrieve them?

Thanks in advance

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