I have always replaced the main BP pic with my own screenshot at the file folder level. Never had a problem till recently. I get an error when trying to publish the BP if I have replaced the photo. The error says "Limit Exceeded" , but I have been told it is because the photo is too large. Is it possible to scale down the screenshot without it looking pixelly? Is it possible to set up my computer to limit the size of a screenshot? I would be happy with the stock BP photo, but on large builds there is an artifact from the control panel screen that ruins the photo....
I think using .jpg work good. I use those at 1440 resolution. You can always scale a photo down without degrading much quality. On windows 10, right click the photo -open with Paint 3D -on the top ribbon click the "Canvas" icon (looks like a tic tac toe game -on the right sidebar you should see a drop menu with "percent" and "pixels" -click on pixels and choose the size you desire. (2k= 2560 x 1440) (1080= 1920 x 1080) (720= 1280 x 720) -then just click on the file folder in the upper left corner - can overwrite the photo or save a new file If that doesn't work you can always use the cropping tool with Paint 3D. I hope this help ya!
Jenn nailed it with the editing. I think the official size it accepts is... 800x600? Paint will do the sizing for you - i.e. choosing the one will set the other. Another great way to help the steam posting is to use steam itself. When in game press the "F12" key and steam takes the screenshot which is automatically sized for their uploading. You can then open that in paint and save it without the shenanigans. Although, if you are pro-shenanigans because you are artist then by all means - I am no artist so I find the shortcut welcome. Hope this helps!