Assuming you have tried a search it might be worth posting this in the COMPUTER help section. There is some info on posting pics (std PC, iPad etc) in that section under athread titled Resizing photos. good luck
The moderator will probably move your post to the correct section.
As daytripper said there are threads with useful tips in the computer section. I use a Mac and have posted photos on here before. It was a struggle but I managed. Use the test forum to try out your efforts and perfect it.
To be honest I just experimented with my computer settings till I got it right. Then when the time came to post forgot it all. Ah old age