Searching for images is an everyday activity. Nevertheless, even a highly skilled searcher often struggles to find what they are looking for. This article studies the factors that affect users’ online web image search behaviour, investigating (1) the use of criteria in making image relevance judgements and (2) the effect of familiarity, difficulty and satisfaction. The study includes 48 users who performed four online image search tasks using Google Images. Simulated work scenarios, questionnaires and screen capture recordings were used to collect data of their image search behaviour. The results show in judging image relevance, users may apply similar criterion, however, the importance of these criteria depends on the type of image search. Similarly, ratings of users’ perception on subjective aspects of performing image search shows they were task dependent. Users’ perception on subjective aspects of performing image search did not always correspond with their actual search behaviour. Correlation analysis shows that subjective factors cannot be definitively measured by using only one component of search behaviour. Future work includes further analysis on the effects of topic familiarity and satisfaction.