The cover image for Profile and Pages in PHPfox Nebula and Neutron do not come with a cropping feature. Due to that, you might want to make a tutorial on your site so that your users know what size image to load. We found that 850 x 315 works best but you can experiment with different sizes and see what works best in your theme as some themes might have different dimensions. For nebula theme, you might also find a smaller size fits best at 700 x 225 as per this post.
You also might want to change the phrases to put the size in it.
To change the phrase
V4 Neutron – Go to AdminCP >> Phrases
Search for the phrase “cover” and you will come up with several selections. The phrase is the same in both V3 Nebula and V4 Neutron. For this tutorial, we will change the phrase for the following cover photo popup which shows for both Profiles and for Pages/Groups so this is a good one to change:
Which is the phrase profile.cover_photo and we would only change the text box in the far right to change this and be sure to click Save All at the bottom of the screen after.
This is how we changed it to add the standard 850 x 315 size:
and how the popup looks after: