how can i resize a picture to 250 x 250?

Also you can try to use picture resizing tools like Batch Picture Resizer (see in comment).

No special knowledge required.

If you have Photoshop, then select the crop tool, then in the tool options along the top type "250 px" in the width and height boxes. Click and drag to make your crop. Hit the enter key to confirm the crop. To save the image click File > Save As, give your file a new name, and select .jpg from the file type drop down. Hit Save.

If you have GIMP - select the crop tool. In the tool options under the toolbox, select fixed aspect ratio, in the aspect ratio box type 1:1. Click and drag to crop to a square and hit the enter key to confirm. Then click Image > Scale Image, type 250 in the width and click Scale. To save the image click File > Export As, then give your file a new file name, and choose .jpg form the file type dropdown, hit Export, then Export again.

GIMP is free http://www.gimp.org/ - Photoshop isn't free http://www.adobe.com/products/photoshop.html