If you are using irfanview...open the pic with it. On the top toolbar, you will see will see an image button. Click it and a sub-menu appears. On the right side, there are several pre-set size options. On the left of that is a numerical guide. You can type in the new size (ie: 250pixels) and it will automatically adjust the pic to 250x150. Go to file and click "save as". Name the pic and click save.