On my hp stream 11s, it's ESCape to get to the menus. Then F9 for the boot devices menu. Cursor up/down and press enter to select. Alternatively pull all bootable drives (or change their contents) to force it to use the only available option.
As far as bootable images, used dd (or dcfldd, basically same with progress info). And use cmp or dd the image off after to check md5sums or contents to verify that it got written. A lot of media types will fail to read only these days. Some almost come in that state from the factory. I have two SDHC cards now in that state. One reached that state within hours of being taken out of the package.