changing sizes of images

I have question about custom field image. I added one to the article then I put some images in items. Images were resizes correctly to specific width/height. Then I changed size parametres in image field but sizes of added images didnt change. If I want images in new width&height I need to remove image from item and put it again. If I had 100 items its a little bit problematic...
Is it a some kind of bug or things cant works diffrent?

Images are resized when they are uploaded, so I think that if you want the system to resize them again you have to upload them again.

You could try creating a macro with a graphic program (phptoshop?) and edit them manually to upload them again in the size you need them to be... for example, you get m_yourimage.ext and resize it so that it becomes s_yourimage.ext with the dimensions you need. I know it wouldn't be quick, but at least it would be faster than manually reuploading them all...