Yes...this is a tremendous feature that Mathieu has added to SHS...As Bastien said just click the + Sign over in the Attachments section of the page on the right. It will allow you to select an Image to upload and below that one can add notes describing what the attachment is (e.g., B-side, Side 1, Back Cover, Disc, Credits...)
1) If you do add Vinyl label images of LPs, please type "Side 1" or "Side 2" to make it easier on the viewer to know what the image pertains too.
2) If you add an image that you don't like you can remove it by clicking on the Edit dropdown Icon at the top of the page and selecting Edit Attachments. It will allow CCs to delete any image (attachment) that you personally added (but not one someone else added), which is very handy when you accidentally upload two Side 1 scans (which I have) or if you realize you need to magnify the image to make it easier to read or you find a better alternative.
3) ADDING MISSING FRONT COVERS/A-SIDES. You also have the capability to add missing Front covers of releases too now by clicking on the Add Information dropdown icon at the top middle of the page (which will only appear there when they are missing). See below images to show you exactly where. The only caveat here is unlike with "Attachments", if you add the wrong Front cover/A-side image or find a better one or whatever it is you don't like about what you have added, you won't be able to undo that at the present time. You will have to contact an Editor to delete that image. So be extra careful on the Primary Release images.