Sorry for the delay.
The file limit is 1MB: https://dev.twitter.com/docs/cards/types/gallery-card
Reading the code, it looks like it’s suggested a 800x800 image. That feels a little large, and there’s other code that says 146x146.
I’d suggest in that range, and the images should appear.
If you’re having other issues, it might be dealing with robots.txt: https://dev.twitter.com/docs/cards/troubleshooting#robots
If you have other questions, please feel free to ask here.
Sorry again for the delay!