How to make precisely sized selections of images in Photoshop (for thumbnails, avatars, etc.)
Something I am working on getting into my website is a thumbnail style image gallery so that when a user clicks a small thumbnail preview of an image, a larger image appears. To do this, I first have to create a thumbnail to appropriately show what the larger image is. You can simply resize your image to thumbnail resolution using Image Size, but I wanted something that showed a detail of the image without showing the whole thing. To do this, start by opening the image you want to make a thumbnail of. Then, use the rectangular marquee tool with a fixed size. Select the rectangular marquee tool, and then change the style in the top toolbar to Fixed Size.
Set the dimensions to whatever thumbnail size you like, usually between 75px-200px on either dimensions depending on shape/size works well for a thumbnail. Click and drag anywhere on the image to make your selection, and then copy the selection to a new document and save*
Hope this helps!
*copy by hitting CTRL+C or CMD+C on Mac, and then CTRL/CMD+N to open a new document. Name it whatever you like, the default dimensions of the new document should be the same size as the copy from the clipboard. Click OK, and then paste the image onto the new document CTRL/CMD+V. Then save the image (CTRL/CMD+S)