alt attribute help Search Engines to understand the image they are crawling.
Alt stands for 'alternate'. It often called 'alt tag' but it is in fact an 'attribute'.
It is important to use alt attributes to describe your images to be in line with good SEO practices. Other image related optimization items are size & load time and image file name.
Image Search still represents for most industry an important SEO subchannel. Given how Image search now works, many Image visits are only virtual and therefore invisible in analytics reports. Image Search is still a fantastic opportunity to improve brand awareness.
<img src="/image.jpg" alt="yourimagealthere" title="furtherinfohere" />
Make sure images are well optimized: from image hosting to image alt tags. In case your website actively uses images, use Image Search to improve brand awareness.
Alt Attribute often called alt tag gives important information to the Google bot regarding what the picture is about. If the image path is incorrect, the alt (alternative) attribute will show instead.
The title appears as tooltip when there is a mouse-over above the image. Use it to give more information about the picture. Although not as important for SEO as the alt attribute, it should not be neglected. Avoid copy and pasting the same text you have as alt attribute.
If your image is too big, this will affect pagespeed, especially for mobile traffic.