Is paying someone to fix LaTeX issues in one's thesis ethical?
I do not see any unethical point in this. 
You were supposed to make an original contribution to your thesis in terms of your research. With respect to writing, you can take help from your friends, supervisor, or anyone else (free or paid). 

In our university in New Zealand, there was a dept. which used to charge a small fee to fix language issues for students with English as a second language. I suppose this should also work for the images/pictures thing as you are not an artist but an expert in your particular field.

Cutting it short, I do not see anything unethical in it.