I’ve noticed this too. I wonder if iNaturalist could do a better job of guiding new users through the process.
I use the web interface, and it was extremely unintuitive to me to figure out how to add new photos to a newly created record. I did figure it out, and I figured out two different ways of doing it, but only after considerable experimentation, and both of them were unexpected. I’m a web developer myself, and I think I tend to experiment more with UI’s than the average user does.
The two ways of adding multiple photos that I discovered were:
(The easy way) Drag and drop an image from some other application open in my OS, like an image program or a file, and drag it into the box for the existing observation that already has one or more images in it. Once I got the idea of trying to drag-and-drop like this, it was intuitive because the box highlights as expected. However, it was not immediately apparent just from looking at the page that it allowed drag-and-drop. The initial page says “Drag and drop some photos and sounds” but once I drag and drop one, there is nothing that says “Drag and drop to add more” and there is no more “Choose files” button. I think the interface would be hugely improved by putting this text and this button somewhere inside the box. There is certainly enough room on the page!
(The hard way) Clicking “+Add” and uploading a new photo and creating a new observation and then combining them, either by selecting both and hitting “Combine”, or by dragging and dropping one into the other.
I strongly suspect a few small UI tweaks would mostly eliminate the problem. Most users aren’t going to fiddle with the interface as long as I did. And even at my level of curiosity, it took me longer than I would like.