I've a webApp to save user picture with description, title. But when I displays all the items the images are big, I want to know how do I set the height and width for the images ?
To do it properly you need to resize the image. Prefably before you put it into BC. While you can for scale the image in CSS you do not want to do this. Why? Because the images are still very big and likely not optimised as they should be for the web.
This means the page with these images will take a very long time to load for people, and people wont be to happy about that viewing your site.
You need to use software such as photoshop if you have it or iphoto on a mac for example to edit, crop and size images before you upload them. If you do not have anything you can use free web based tools such as this one:
You can also do this within BC itself in the file manager. If you click the image you can see options to resize it. The downside of that is that is done by pure math. It does not know what is in the photo and the results can make the image look squished etc.