Be mindful when you name your images before you upload them to a website, especially one that you cannot control, like a social networking site or a blogging site that asks for a profile picture or logo. This way, if a website doesn't let you write an alt attribute for your picture, your filename could do the same work by making a keyword recognizable. The alt attribute is the text that appears in the place of an image if the image is broken, or if a visually impaired person has settings to have alt attributes read out loud.
A social networking site may automatically fill in the alt attribute with your filename. Even if you cannot access alt attributes from a site that allows you to upload an image, it may automatically write them for you. How do it know? Usually a website system will create it directly from the filename of your image.
For example: if you have an image called "logo-483.jpg", that doesn't say anything about your company name. But, if your filename was called "logo-your-company-name.jpg", that has more of a chance of coming up in a Google Image Search result. This is a minor action you can take to affect your SEO, but it can have a great impact.