You can upload picture from any RichEditor field. Copy that uploaded image url and then write html code in Views > Shared > Components > Footer > Default.cshtml file.
Yes, you can save those files inside wwwroot folder. Let you saved that in wwwroot > images > myFolder directory. Then write html code like below in Views > Shared > Components > Footer > Default.cshtml file.
Put your image inside wwwroot > images > uploaded folder. Let your picture file name is Untitled.png. Then add this HTML code in Views > Shared > Components > Footer > Default.cshtml file.
<img src="/images/uploaded/Untitled.png" />
Note: you can put in any those folders (flags, sample, thumbs or uploaded). Even you can create a new folder. In that case you need to change image source path (src) in HTML code.