I've clicked on an image, but now it's not visible anymore!

Yes, the image will disappear when clicked on. I'd recommend you to test in Chrome in cognito. You can also try another browser or deleting your browser cookies.

Can I apply the popup to a normal image block?

Yes, you can. The add-on adds an extra image block template. However, using the custom block is recommended.

Is this add-on compatible with concrete5 v8?

Once v8 is released, this add-on will be made compatible.

What do the placeholders %FOUND% and %REMAINING% mean?

%FOUND% can be used in the text and will be replaced by a number. The number of items that have been collected by the visitor. %REMAINING% is also a number and displays how many items still need to be collected. For example:
POKEMON challenge! (%FOUND%/%REMAINING%) --> POKEMON challenge (4/10)

What's the cookie name about?

By default the cookie will be valid for one year. Meaning that the 'quest' will be completed for a visitor. If you want to reset the quest or create a new one, you can rename the cookie. The default cookie name is 'treasure-hunt'.