<img src="image .jpg" alt=""> here what is use of alt=""
If your browser don't found the image due to mistake to path or the picture deleted from this path, the text between the quotation will appear in place of the image. e.g. alt="My photo" will display My photo.
If your image is not able to load or someone is using TEXT ONLY mode on their browser, the text between the quotation (alt"") will appear instead of your image.
If i can remember this i think alt= stands for a default picture setting but im not clear that that is the answer