You should assume that everything you find on the internet is copyrighted and cannot be freely re-used, unless there is an explicit statement that says it can (these are often found in the Terms & Conditions section), or there is clear evidence of a licence, such as a Creative Commons licence, having been applied.
You can however make use of the education exceptions outlined in Sections 32-36 of the Copyright, Designs and Patents Act 1988 to re-use content from the internet.
NB. Some search engines allow you to apply filters to search results, e.g. to filter by items ‘labelled for reuse'. However, these cannot be relied upon to provide accurate results, so you should always follow the link to the source website to double-check re-use conditions yourself. Alternatively, if it's an image you're after, click here.