Can I move my pictures from my computer to an external hard drive / delete the pictures from my computer/ and at some time in the future bring them back to my computer. If I can how do I do this ?
Yes, easily. Any external hard drive once connected appears to your Operating system and behaves exactly like any other "normal" internal drive. So it would be easy to Copy whole folders (including all its files and subfolders, and their files etc.) from your current internal drive to any external unit that you connect. Then you could delete from the original drive. If you want to actually free up that space to use it, don't forget to Empty your Recycle Bin.
At any time in the future you can simply reverse the process for all or part of the files. In the meantime, at any time you can simply use the external drive (if it is connected and turned on) the same as any other - load a file, use it, edit it and store again, etc. - directly from that drive - no copying to the internals necessary. The only risk / limit is exactly the same as for any internal drive - how many years will the data last on the drive once written there? In fact, if you disconnect the external drive unit from the wall electrical supply and your computer when it is not in use, you will protect it from some potential problems and lengthen its mechanical life by not having it running when not actually using it.