Players could choose inappropriate pictures, they could also choose pictures to bully other players (100% convinced this will happen all the time). This will require more moderation time that could be spent elsewhere.
This is also possible with normal pfp's...
And almost nobody really does this. Not a good argument.
This will require more computing power from phones and it will also use more data to load these custom images, especially if they are big, this will make the forums less performant on phones, which could reduce the daily visitors.
It will also require more server space on the forums because the images will need to be hosted online by CubeCraft for loading. This could increase load times across the site.
This, by contrast is a good argument. I do think this small change isn't worth all the extra loading time on phones, and as you mentioned, it probably will decrease the daily visitors because the forums will be used less on phones. (Because of all the loading etc.)
So in my opinion, it is a nice suggestion, but as Story mentioned, it will cause more loading time for phone users.
So... it's a no for me. I'm sorry.