You could set the resolution to a very large dpi value but this will not modify the image in any way except that the image would be printed smaller and thus have more pixels per inch. The ppi dpi value of an image file is just a number embedded in the header of the file suggesting the number of pixels to be printed in one inch. Each one will appear in its own tab inside the photoshop interface.
You can open more than one photo at a time. Double clicking on your desktop right clicking and selecting open with choosing file open from within photoshop and so on. You can open an image in photoshop in the same way you open any other file in any other program.
For example in electrical engineering a 2 watt component will handle two times the heat of the same 1 watt component. There is no way to add 10 more software to improve its stiffness or performance margin. While it is possible to add a brace or bracket to a mechanical design to address a design shortcoming software has no such complement.