Chain pops off guide pulley when pedaling I re-installed new tires for the first time yesterday. Now I'm having problems with my rear derailleur. The chain keeps popping off the derailleur guide pulley when I pedal forward. I can re-seat it easily but it just keeps popping off. Pics (and a video) below.

Disclaimer: this is a piece of crap bike that I got for free so thats why it looks like garbage. Also I am new to bike repair so I almost certainly did multiple things wrong
I think the chain is not tensioned enough. Since you did not change the chain, or the crank, just the tires, the wheel position seems to me to be the culprit.

From the last picture it looks like the axle was farther behind (see paint chipped ... or at least I am seeing it, maybe it is an artifact?)

So, release the nut, slide the wheel more to the back in the guides and then close the nuts. Setting the gear in the small rings (back and front) should help you pushing the wheel back.