while designing a webpage the criterias to be kept in
As Instructional Designer one has to keep a track of all the things and the placement of graphics and imsges. Now I would like to know how one can decide the placement of a particular graphic within in a content.
This question doesn't have one set and done answer. It all depends on what the graphic/image is and the layout of the website. This is where your artistic ability has to come in. If you could show us the graphic/image and the webpage that you are trying to put it on we may be able to help you then.
Every graphic is either a jpeg or png so it has a specific measure , inorder to run it smoothly it should be edited in photoshop and its texture should be changed inorder to be attractive such that it suits the given content and it looks highly acceptable.
Graphic and images both are static in nature hence they can be modified easily in photo edited softwares as per the given content but incase if they are animated then one has to adjust the animation as per the size of the web page and it must be soothing to he eyes of the viewers.
Ah, Instructional designer, what is more important graphics and images or the content. And what supports what? How much content do you have, what media/program u are using to present your content, and whether you can give thumbnails that can be enlarged etc are the issues u need to consider.
Sometimes one can ramble on pages after pages of verbal content without any pix. That can get boring. There are may topics which need a image/graphic which are refered to and explained in the content, in such cases the image must be legible and details should be clear. Also bear in mind who is your target audience, will they have adequate bandwidth to download? If most of audidence is using dial-up services, keep the image size minimal.
while designing a webpage the criterias to be kept in 
Linear Mode
