Hi! I’m not entirely sure if the answers I’m going to give are perfect since you’re working with a school firewall, but I hope one at least works for you!

You can use Dropbox for image uploading — this is usually a safe option since a ton of schools out there utilize Dropbox for assignments. Alternatively, you can use Google Photos too. However, with these options, keep in mind that you’ll have to fish for the direct link of an image by right-clicking and copying “image address” for the full extension to use here with the image code!

Deviantart has a feature called Sta.sh! I’ve seen a lot of people use that to upload files for the purposes of image hosting.

Creating a private Discord is another option for image uploading and hosting. The downside is that it’s often a hit-or-miss thing whether a school firewall has Discord blocked.

The last option I can recommend is a website called tinypic; while this is an alright alternative to the options aforementioned, tinypic has a history of deleting old image uploads. That can understandably be a bit of a handful to deal with.

I hope one of these works!