The images are automatically uploaded to the media area.
Wix also already optimizes all images and converts to webP format. You can read more about images here. https://support.wix.com/en/article/site-performance-optimizing-your-media
"Wix automatically optimizes your images for best online quality and fast download. This means that, unless your file is over 25MB, you do not need to resize or compress it before uploading. "
Again, if you want custom functionality you will need to build your own forum solution. You will not be able to divert the images to another location if you choose to use the forum app
Edit: This is not true of all apps as some have more places to customize the ootb functionality, but the forum is not open in this way so you kind of need to choose to use it as is or roll your own version which is possible on Wix but may not be your best use of time depending on your goals.