It is a bit hard to tell without the image files and their filenames. Adding them to the question would help a lot.
From Q2A's point of view you can only upload png, gif, jpg and jpeg files. Note that both, the extension and the actual content of the file is checked to match those formats. The plugin you're linking, at first sight, seems to perform a similar check to the one Q2A performs to make sure the content of the file matches one of the allowed formats. This means, no additional check seems to be added.
Q2A also checks file size. Opposite to what it would seem at first sight, this can be considerably complex stuff because there are limits in application logic (Q2A) and in Apache/PHP. In Apache/PHP, there are actually 2 checks: maximum attachment file size and maximum post size. Here is a related question.
If these approaches don't seem to be the cause of the issue, consider uploading a couple of images in order to be able to reproduce the issue. If you can also include your Apache/PHP settings, that's even better.