The issue is probably the resize code you're using. Subeta uses sCode and doesn't support all BBCode. If the image needs to be resized, you'll want to actually resize the image (like with an image editing program) before uploading it to imgur and using sCode to display it. You can find our sCode reference here.

Any image editing program can be used to resize the image. I'm not aware of online resources for that specifically - I've never had the need - but any image editor for your device (computer, phone, tablet, whatever) can typically be used for cropping or resizing.