For 80% of the time I want my Photoshop images "saved for web" to be of the file type
png-24. I know it is possible to set it as default, for I did it once before, but I don't know how to do this.
Right now it always prompts to save as JPEG.
It should be setting a flag for your last used type of save... so even if you close the app, then reopen it, create a new file, then save for web... it SHOULD goto the last used type of save.
But this is Adobe software.
So there's a cache of some sort that might need to be flushed, a reinstall might be required, and there's probably pending updates and fixes due.
If this is not working, you can create an Action that gets you to the part where you specify the save location with it already to go as a PNG-24.
Actions are probably the coolest part of Photoshop.
You record them in the Actions panel. Just open an image you want to save, press record on the Actions panel, Do "Save for Web" from file menu, then save it somewhere. Then press stop on the record.
Now you're going to need to put a "break" into the action right before it saves, so you can now manually specify the location and file name.
And the Action can be made into a button and/or assigned a keyboard shortcut unique to itself.
// Think of Actions like step based Macros, if you've ever used them in any other kind of app. You can make them INCREDIBLY complex.