1. BEFORE YOU SPEAK
Look at the picture(s) for a while and think - identify objects/people in the centre (they are usually the main topic => they represent the area of vocabulary you will need), identify general topic (e.g. holiday, winter, modern life style) and think about mood of the picture. Also (in case of a group of pictures) think about ideas they share or differ in. The last think to do is to choose a picture or two - you will have to describe them in detail later.
2. SPEAKING PART
always use some kind of introduction sentence. Start talking about general things - go through objects in the picture, surroundings, overall impression and then identify the main topic. Describe details of the picture (people, their appearance, age, activity, clothes, feelings, relationships, their surrounding, weather conditions etc. Finish your speaking with some personal experience or memory connected with the topic. In case of two or more pictures compare them to each other and talk about the topic of the whole group. Try to think about their message to conclude.
3. EXPRESSIONS, GRAMMAR AND VOCABULARY
During your speaking use ordering expressions like 'first of all', 'now' and 'finally' (for introducing individua;l parts of your speach), use expressions of opinion making 'in my opinion', 'in my view' or simply 'I think' and expressions for connecting ideas 'and', 'but', 'because', 'however', 'although' etc.
Grammar expected in picture description is: the present continuous (beware the state verbs) and 'there is/there are' phrase. These two are a MUST. Then you should also think about using articles. If suitable, you can use any other grammar you like.
Vocabulary is open - it depends on the topic but basic expressions for everyday life situations are another MUST. Also you should be able to describe position of things in the picture (=in the foreground/background, at the top/bottom on the right/left, in the corner ...). Generally, always try to impress with unusual vocabulary.