If you have a Mac, then you don't need Photoshop to prepare your photos for the web. You can use the built-in Photos app or the Preview app, which is the one covered here.
In this quick tutorial, we're going to resize and convert some photos of different widths in .TIFF format to .JPGs with equal widths.
I use ⌘ symbol for the cmd (command) key, and the ⌥ symbol for the option (aka alt) key.
Tip: You can see the size of an image by selecting it, then typing ⌘i.
Note that Preview is "trigger happy".
Anything you do is live, ie. it is saved immediately. So preferably duplicate the photos you intend to work on by selecting them or their containing folder, and typing ⌘d.
Open the photos in Preview.
They should be of the same format. Sometimes Preview will split different format into groups, each with its own window.
Drag over the photos in the sidebar to select them all.
Or click in the sidebar to (invisibly) move the focus there, then type ⌘a.
Go to the Tools menu, and select Adjust Size...
Enter the size:
Go to the File menu and Export Selected Images...
In the save dialogue box, click Options near the bottom left.
Select the file format. If JPEG, then make sure they are at a reasonably high quality setting:
Choose the destination to save them in.
Technically we're done at this point, but this last step is a must.
Get a copy of ImageOptim, and install it. Launch ImageOptim, and drag your newly-processed photos into the ImageOptim window.
When it's done, your photos will likely be anything between half to 1/10th of their original file size.
This means that after you upload your pics, your website will load very quickly.
Google will love you, and you'll go places.
Yes, just because you did that last step.