This is definitely an interesting idea, and one with some good potential.
I feel like the biggest oversight is that images aren't resized at all. This means that when using high resolution images, the player will only see a small portion of the image. Having something in place to scale images so they fit within the given resolution would be - I would think - a very high priority.
A much smaller area of improvement could be the overall UI - the main menu looks fine enough, but adding a bit of style to the actual game would be nice - for example, maybe the tempo bar at the bottom could have a border around it, and then you could make it so the background within that box flashes on beat. That last bit isn't really important, just a random thought I had.
Another random thought I had - something that is very *low* priority as far as I'm concerned - would be a "music mode." It'd probably be very complicated, but being able to insert a song, and then have the tempo match up with the song would just be kinda neat. That's probably pretty difficult to do though, so again, not really expecting that to happen. Just an idea. However, having *something* in the background other than the clicking might be nice.
The last thing I'd like to suggest would be the ability to pull images from various online sources directly. I think in this day and age, a lot of people keep their, ah, "collections" online, and having to download six or seven thousand images is, ah, a mild inconvenience. Being able to somehow link accounts and have the game pull directly from that - or just being able to use a search function to pull images from the site based on the search - would be a very quality-of-life thing to add. However, I have no clue how complex or simple it would be to add, so again, if it isn't added, oh well.
But at the end of the day, for what this is, it's pretty well made. Good job!
