1)I want to find out if any text,image or anything exist in the region in which i am inserting my image how would i do it ?.
Insert an image and see by yourself what happens. If you are allowed to or text gets replaced.
2) Is there any problem if i insert the images in parallel loop it will increase or decrease the performance?
Now, that would be difficult for one to tell until a lot of image/data insertion happens. Once, again, just do it and see?
If you have to discuss or get more information after trying out then post at iTextsharp forum and get their expert opinion about it. Specific questions related to 3rd party can be answered more quickly and accurately at their forums: