i want to create a image[asp.net application] with another image as a canvas.

Now what i would like to have is, instead of asking the user to upload the canvas image, is it possible to get that image from another url, something like

http://www.somedomain.com/canvasImage.jpg

Yes it is possible.

You will have to use HttpWebRequest and HttpWebResponse Object to get image stream from remote server and then use Bitmap constructor which takes stream

However using image from other site without permission can you in trouble.

we are planning to have the users upload images and use them as canvas images for creating their own images, and we dont intend to store any of them in our servers. so we think we will not end up with trouble with this structure.

can i have a sample code for the HttpWebRequest Method

Copied from http://msdn2.microsoft.com/en-us/library/system.net.httpwebresponse.getresponsestream.aspx

No, Browser can not get file size on client side (Security reason), You will have to uploade file on server to get file size.

However JavaApplet or ActiveX control  can do that job.

miztiik:

Not necessarily in the broswer. once we start geting  the first few bytes of the file, cant we determine the size of the file and proceed further accordingly?

You will have to wait untill you receive bytes that are more then your set limit and then stop receiving further, its not out of the box feature but you can do it with HttpModules.

Also try this open source upload control http://www.brettle.com/neatupload it comes with source code which you can modify to suite your requirment.