Hi. To upload images you just need to be logged in. You also should have:
1. a clear and unique name for your image (you cannot overwrite other people's images). the naming convention is name-category-wiki-guide. sword-weapon-dark-souls-wiki-guide.jpg for example
2. use .jpg instead of .png unless the image is transparent
3. use sizes that match the objective. If the image is a thumbnail, make a 300px or under version of it rather than upload a 4k image
4. keep under max upload size from the editor
If you're having trouble with a specific image please upload it to imgur and post it here so I can have a look.