Hello. Google indexes my images very quickly if I just give the direct links. But sometimes, I have very larger images and they don't fit to my page so in that case, Wordpress automatically resizes that image and posts that as a thumbnail while linking it to the original size. But I see, Out of 50 such images, Google just detected one of them and that was also the thumbnail ( resized ) image link. I heard that Google infact likes when the images are linked properly. But why doesn't it index that properly ? That's one way. Another way but time consuming one is that, I can just put full size image and manually specify the height and width to fit my site. Which one is better ?
As far as i am concerned Google mainly looks for alt attribute while indexing images so make sure its not missing specifying the height and width of the image is for making the page more presentable but, it leaves almost none or little impact on seo You stated that you are using wordpress so, i think you better leave the things to default because wordpress is well known for its seo friendliness
Exactly - the size of the image does not matter at all. Make sure you have the "alt" text containing your keyword, and name your image accordingly (have the keyword in there, too!).