I will use a reference image throughout, an MG Midget with a couple of people standing by it: mg.jpg. This is a jpg image, with dimensions of 1024 x 683, file size of 519kb

1. Resize an image

convert mg.jpg -resize 500 mg.jpg

This resizes the image to 500×333, (maintaining the aspect ratio)

2. Crop Image

convert mg.jpg -crop 150x125+350+350 +repage mgcrop.jpg

You need to specify the output size (150×125) plus the start position for x & y on the original photo (+350+350). It may take some trial and error to find the exact spot! The +repage setting removed the virtual canvas (blank space left of original image after cropping).

To help things you can use a couple of IM tricks: gravity and percentage.

convert mg.jpg -gravity Center -crop 150x125+0+0 +repage mgcropgravity.jpg

convert mg.jpg -gravity Center -crop 50%x+0+0 +repage mgcropgravity50percent.jpg

3. Add Borders to an Image

I have used white borders so they show in the blog but you can use any colour your like:

convert mg.jpg -bordercolor white -border 20x20 -resize 250x mg borderx20resizex250.jpg

You can also use a percentage for borders:

convert mg.jpg -resize 250x -bordercolor white -border 20%x20% mgborderx20percentresizex250.jpg

or

convert mg.jpg -resize 250x -gravity center -background white -extent 120%x120% mgborderx20percent.jpg

4. Make an Image Square, with a Border

convert -define jpeg:size=200x200 mg.jpg -thumbnail '100X100>' -gravity center -crop 120x120+0+0\! -background white -flatten mgsquare.jpg

5. Make Thumbnails

convert -define jpeg:size=200x200 mg.jpg -thumbnail 100x100^ -gravity center -extent 100x100 mgthumb.jpg