Users are given 700 PMs storage which really should be enough for the vast majority of individuals who are not vendors. Appreciate the thought with donations but we currently do not have in place a pay for more storage option.
The reason we limit file size and resolution is because it bogs down site resources, particularly with photo heavy threads. It's a balance with having a pleasant user experience. Max file resolution/size is at 1900x1900 and 1.9MB max. We can increase it to 2.4MB if that would help but really not going to push it much more than that.