Checking out the thread you've made, you have the edge to compete with those professional thread designers themselves at the services section.
And I like the color combination of that example thread you've given. Are you depending to the color that was demanded by the company or you are the one that's deciding for them?
Trying to help you out with the size of image per thread but I haven't seen on the meta section that there's a thread for that or I didn't just dig the latter part.
Thanks. Regarding the example thread I linked and your questions.
I decided on those colors. Usually at the start of a project I ask clients about their preferences (i.e. dark/light layout), if they already have existing graphics or if there are certain colors they'd like to work with, etc. Most of the time I'm not given specific requirements with complete freedom to design something.
Also couldn't find anything regarding image limitations myself, so I don't think there are any real limitation.
Except that you have to be Jr. Member to post images if I'm correct.