Try dragging and dropping a picture in a worksheet - What happens. You will notice a plug sign which means that a picture will be added. But when you release the cursor to drop the picture in Excel sheet nothing happens.
Reason - Excel doesn't support dragging and dropping the picture in a worksheet.
Now, what to do. If you are dealing with too many pictures, it is cumbersome to use Insert > Picture command.
Fortunately, there is a work around in place. Microsoft Word supports dragging and dropping of pictures. Also good thing is that, you can select all your pictures in one go and drop in Word document.
Now, you can either drag and drop the pictures from the Word document in Excel sheet or you can cut/copy and paste from Word to Excel sheet.
Side Note - OpenOffice / Libre Office supports drag and drop to their respective spreadsheet programs. (Note, but you can not drag or copy and paste pictures into Excel from Open Office / Libre Office spreadhseets like you can drag or copy / paste from Word)