Do you have a favorite software for editing your photos? It might help to know your workflow in order to advise you appropriately. Many programs have the ability to do this. Once you settle on a particular method, it's helpful to set a routine were you export images you intend to share as smaller version, usually around 1200 to 2000 pixels on the long side, so you're not uploading full sized jpegs, and put them in a cloud storage folder, like Dropbox. I have a folder called Social Media that I put them into and from time to time I cull that folder so it doesn't get too full. That way you can access the images from a smart phone, another computer, a tablet, etc. For creating the watermark, I contracted with Photo Logo to create my own 'signature' style logo. I use that file within a program from Adobe called Lightroom, to add the watermark if and when I need it to be added. I don't use it very often, but it's handy to be able to help people recognized the author of the photo when shared online and build name recognition perhaps.