Startup disc creator not allowing any iso to be loaded

If the .iso file was downloaded as by root for some reason it could be unreadable by your regular account. You can open a terminal with Ctrl+Alt+T and enter the commandsudo chmod a+r the-image.iso to fix that.

I assume you are using usb-creator-gtk, the standard Ubuntu tool for this. I've had some trouble selecting the iso file I want with that program because of the small window available to select files. Let me suggest starting it from a terminal by using

usb-creator-gtk -i the-image.iso

This was solved for me by removing any .img files from the folder where the desired Ubuntu iso is located. So basically, I had an Android boot.img inside the same folder as the Ubuntu 12.04.1 iso, which is what I wanted to use for startup-disk-creator. I moved the boot.img to a separate folder, re-opened startup-disk-creator, and it worked. I don't know if it's a quirk with specifically choosing img files over iso files; or if there is a general issue with having two or more startup-disk compatible files inside the same folder, I just know that isolating the Ubuntu iso from other types of startup files solved the issue for me.