Are you losing work in Construct 3's Animation Editor often? This may be your problem.
At first and for quite some time, I've noticed (along with multiple other people) while making an animation on construct there is times that pasting an image would not work. It would seem like it worked in the editor but after closing the editor, the image would be blank.
I simply did not understand it and always considered it to be a bug that would begin to persist.
I've finally figured out the cause of this and I'm not sure if it's a feature I am just not understanding, or if it is just a bug.
It seems like when ever you put "invisible" pixels on the image (excluding the erase tool and the fill tool) the image you are pasting is removed/subtracted from the image in the editor. There are times this may or may not be immediately noticeable, IE: some times the image will be invisible as soon as you paste, sometimes it looks normal, but it will always be gone from the image in the frames window.
This was a problem for me because I usually use the rectangle tool to erase everything.
This can be utilized if you want to subtract an image from another image I suppose... but it mostly seems like a hinderance while trying to make/edit an animation in the construct animation editor. If this is a feature, please show me something that documents it all or in general it should be made more noticeable because it entirely seems like a bug to me.
To recreate this:
-Create new project (or try it in your own project)
-Create new sprite (or try it with your own sprites)
-(if new sprite) give frame 0 an image
-Copy frame 0 with select tool or Copy (ctrl + C)
-Use any tool other than Eraser or Fill to put down "Invisible"(0 Alpha) pixels (using rectangle or Ellipse/Circle will not recreate this if either of the fill or border is not also 0 Alpha)
-Paste the image - It will remove the image you were pasting, so if it's the same image, it will be gone. If there was some difference in Alphas in the pixels of your pasted image, it will subtract the pixels Alpha. If you paste a smaller image in, it will cut it out.
NOW that I spent the time to figure out the cause of this, it seems so trivial and easy to avoid, but for a while I did not understand it and it bothered me and costed me a fair amount of work. I'm sure there are other people out there who do not understand it and constantly lose animation work, so hopefully my post is found by them and this can shed some light on the situation.