I use a lot of images in my work and generally I need to be able to find the right image quickly and move swiftly with them.
Because of this, my favourite image resource is a paid for resource. Freepik.com is only $9.99 per month and allows me to quickly and easily find high quality images via a non attribution licence. For ad hoc work, I can set up my monthly subscription then immediately cancel the subscription with access remaining for the full month (therefore not getting stuck in a monthly rolling subscription).
If I can’t find them on Freepik, other free image resources (these will mostly likely require attribution under creative commons licences) I find useful are listed below:
For general image searches: www.freeimages.com
For vectors: http://www.123freevectors.com/
For icons: http://findicons.com/
And if you just can’t find what you’re looking for check out http://www.bluevertigo.com.ar/bluevertigo.htm for a full list of image resources.