There's no silly questions but..... 
I can see the drag and drop option but can't see a simple - import - select folder- select file method.
[edit]
As I was typing this I decided to try the File - Import option and saw that it can import costumes so basic issue solved
But, that method didn't leap out at me to use for a costume import
Could I make a feature request to have a button next to the other costume buttons please?
[/edit]
Hello!
I don't know if there's an easier way to go about this (other than drag 'n drop, of course), but if you would rather search through your files, you can hit the paper-looking icon in the top left, then hit "import", and select your costume.
It should then get imported as a different sprite, then you can just drag and drop the costume from that new sprite over to the desired one.
Drag 'n drop is much easier though, so I'd recommend you continue using that.
If the costumes tab is open and you import an image, it adds it to that sprite. I never figured out how to drag and drop, so I just use that.
"Import" checks the file type, so you can import sprites, costumes, sounds, libraries, text, ...
(You export a sprite by right-clicking its icon in the sprite corral. The exported sprite includes not only its costumes but its sprite-local blocks, its scripting area, the works.)
PS About the button in the costume tab, yeah, I agree. But it's not urgent so don't hold your breath.
It is not working for my Snap! I am using a tablet. I am trying to upload multiple alonzo costumes but when I press import and choose the file, it is not working.
Is there a problem with my Android Chrome, or is it another issue? 
Nothing happens? Or, the costume import window closes without importing? Or, the costume is imported but the window stays open? Or something else?
I am trying to upload alonzo svg costumes on a Samsung Galaxy Tab.
And WHY IS THE FORUM NOT ALLOWING ME TO UPLOAD THE COSTUMES!?
You trust level is a new user, which has restrictions on posting images. After you have been on the forum for a while, you should be able to become a basic user to post images.
Is that the same reason that I cannot import images onto Snap!?
No.
I am selecting the image, but when I select it, it nothing happens.
I have tried this out on my Android Tablet
I emailed myself a copy of the Alonzo svg and downloaded it to the tablet.
Opened up Snap! in Chrome and selected Import
Selected Files folder
Clicked on the Alonzo costume
It was imported into Snap!
I'm on Chrome 122.0.6291.90 on Android 12
I use Chrome 122.0.6261.90
Just as an experiment - can you import the costume file into a Scratch project?